Conventional wisdom has it that training with high reps and light weights builds endurance, but makes little contribution to gains in muscle mass.
Heavy weights and low reps has long been the accepted “best way” to maximize muscle growth.
Back in 2010, Stuart Phillips, a kinesiology professor at Canada’s McMaster University, began to overturn much of that conventional wisdom.
Phillips and his team found that muscle protein synthesis, a key driving force behind muscle growth, was higher with light weights and high reps (4 sets of 24 reps) than it was with heavier weights and lower reps (4 sets of 5 reps) .
At the time, these findings generated a lot of controversy.
They were also dismissed by many, mainly on the basis that the study looked at short-term changes in protein synthesis rather than long-term gains in muscle size.
So, Phillips set up another trial.
This time, he got a group of guys to train their legs on the leg extension machine three times a week for 10 weeks, using one of three different set and rep configurations :
- 1 set of 10-12 reps performed to voluntary failure
- 3 sets of 10-12 reps performed to the point of fatigue
- 3 sets of 30-40 reps performed to the point of fatigue
The amount of new muscle added to both legs was almost identical. Training with lighter weights and higher reps stimulated just as much muscle growth as heavy weights and lower reps.
What’s more, the average size of both the fast and slow twitch muscle fibers increased equally with heavy and light loads, meaning that both fiber types were recruited and stimulated during training.
Once again, this study attracted criticism, most notably because it used untrained beginners as subjects.
Take someone who’s never exercised and get them to lift weights for a few months.
They tend to grow no matter what they do.
Are you going to see the same results in guys who have been training for a few years?
To find out, Phillips recruited a group of 49 men with an average of four years lifting experience. The men were assigned to one of two groups . One group did 20-25 reps per set, while group two did 8-12 reps per set.
The findings: After 12 weeks, there was no statistically significant difference in the rate of muscle growth between the two groups.
As with the research in novices, the average size of both the fast and slow twitch muscle fibers increased to a similar extent in both groups.
But, that doesn’t mean the two protocols delivered identical results.
The average amount of muscle mass gained in the high rep group was 2.2 pounds (1 kilogram), compared with 3.5 pounds (1.6 kilograms) in the low rep group.
Had the study lasted longer, or the number of subjects been larger, the difference in results between the two groups may have become large enough to cross the statistically significant threshold.
Of course, these are the results from just a few studies. As I’ve explained here, drawing conclusions about anything from two or three studies is never a good idea.
However, there’s plenty of other research out there showing multiple benefits of training with a light weight and high reps.
- Light slow-speed training (55-60% of one-rep-max, 3 seconds to lift and lower the weight) has been shown to increase both muscle mass and maximal strength . The results are comparable to those obtained with heavy normal-speed training (80-90% of one-rep-max, 1 second to lift and lower the weight).
- Both heavy (4 sets of 8-10 reps) and light training (4 sets of 18-20 reps) activate the expression of various genes involved in muscle growth .
- Eight weeks of training the arms with light weights (20 rep-max) and short (30 seconds) rest periods led to gains in muscle size that were no different to those seen with heavier weights (8 rep-max) and longer (3 minute) rest periods .
- Training with lighter weights and higher reps (not to failure) also stimulates protein synthesis in connective tissue just as well as heavy training, giving it a role during injury rehabilitation to improve regeneration of connective tissue .
Two Types of Muscle Growth
Some say that the type of muscle growth caused by training with lighter weights and higher reps isn’t as “good” as the muscle gained with heavier weights.
Here’s what they’re talking about.
If you could take a closer look at a slice of muscle tissue, you’d see that it’s made up of many smaller muscle fibers. Your muscles get bigger when these individual fibers become thicker, a process known as hypertrophy.
Inside each fiber are rod-like structures called myofibrils, which run parallel to one another. Myofibrils are the part of the muscle that contribute to force production.
There is also a fluid part of the muscle fiber, known as the sarcoplasm, in which the myofibrils are embedded. It’s filled with stuff – such as water, glycogen, and myoglobin – that doesn’t contribute directly to the production of muscle force.
The term myofibrillar hypertrophy refers to an increase in the volume of the myofibrils, while sarcoplasmic hypertrophy describes the expansion of the sarcoplasm.
The idea is that training with lighter weights and higher reps leads mainly to so-called “non-functional” sarcoplasmic hypertrophy, creating “puffy” muscles that tend to deflate relatively quickly.
Training with heavy weights and lower reps, on the other hand, is said to preferentially increase the rate of myofibrillar hypertrophy, leading to dense, strong, “functional” muscles.
That’s the theory, anyway. Personally, I’m not convinced.
In one of the few studies to compare the effect of high versus low repetitions on myofibrillar and sarcoplasmic protein synthesis, low weight high reps training did increase the rate of sarcoplasmic protein synthesis to a greater extent than heavier training .
But, it increased the rate of myofibrillar protein synthesis too.
Interestingly enough, when it was measured 24 hours after exercise, high reps and light weights (4 sets of 24 reps) increased myofibrillar protein synthesis to a far greater extent than low reps and heavy weights (4 sets of 5 reps).
Granted, this study looked at short-term changes in protein synthesis after exercise, rather than long-term changes in muscle tissue after several months of training. We can’t assume that the former is a completely dependable way to predict the latter.
But it does paint a big question mark next to the idea that training with higher reps and lighter weights will preferentially increase sarcoplasmic rather than myofibrillar hypertrophy.
This doesn’t mean that training programs built on the sarcoplasmic versus myofibrillar hypertrophy concept don’t work. Just that the way in which they’re supposed to work is probably wrong.
The sarcoplasm exists. It can grow.
But there’s no convincing evidence to suggest that gains in muscle mass produced by training with lighter weights and higher repetitions comes predominantly from “non-functional” sarcoplasmic hypertrophy, while the myofibrillar component of the muscle remains the same size, or grows to a much lesser degree.
What does all of this mean for you?
It’s not that training with light weights and high reps is now the official “best way” to build muscle mass.
The fact that it’s possible to gain muscle using higher reps and lighter weights doesn’t necessarily mean that it’s a good idea to do so.
Remember, there were no discernible advantages to the light weight high rep protocols. They didn’t lead to superior gains in size or strength. But each set took twice as long to complete.
Training to failure in a higher rep range is also highly unpleasant and extremely painful – a lot harder than lower reps and heavier weights.
Doing longer, more painful workouts simply to generate the same results doesn’t sound like a great idea to me.
Plus, lower reps and heavier weights still win the day as far as gains in strength are concerned .
What it does mean is that the range of repetitions you can use to build muscle is a lot wider than previously thought.
Most studies out there show very similar gains in muscle size across a variety of rep ranges.
That gives you a lot more choice about the type of training you do.
For example, joint issues or injuries may mean that lifting heavy weights causes pain in your shoulders, elbows, knees or wrists.
The solution is very simple. If going heavy on certain exercises causes you pain, just go light instead.
You can make the switch from heavy weights and low reps to low weights and high reps without missing out on any gains.
Maybe you just prefer using lighter weights on certain exercises, and heavier weights on others.
Again, you can do so quite happily without worrying that you’re putting the brakes on muscle growth.
In short, as long as you train hard and push yourself, muscles can be made to grow with a variety of rep ranges and weights, from light to medium to heavy.
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ABOUT CHRISTIAN FINNChristian Finn holds a master's degree with distinction in exercise science, is a former personal trainer and has been featured on BBC TV and radio, as well as in Men's Health, Men's Fitness, Fit Pro, Zest, and Perfect Body magazine.